Method of reducing the sodium sulfate content of halite

Abstract

The content of sodium sulfate (and less soluble sulfates) in rock salt (halite) can economically be reduced from levels as high as 5 weight percent to levels as low as about 0.1 weight percent by the process of the invention. The halite is crushed to a certain particle size range and particles less than 0.5 mm in effective diameter are removed while (or after) the crushed material is subjected to attrition washing with a low sulfate, high NaCl brine. The washed, coarse particles are rinsed with a low sulfate brine, drained and dried to an extent appropriate to their contemplated use.

What is claimed is: 
     
       1. The method of reducing the sulfate content of a halite, which comprises: (a) providing pieces of halite which have effective diameters greater than about 2.5 millimeters and are aggregates of sodium chloride crystals, said aggregates containing from about 1 to about 20 volume percent of saturated sodium chloride brine and from about 0.5 to 5.0 weight percent of sulfate bodies which have effective diameters less than about 0.5 millimeters and are embedded in or between said crystals,   (b) crushing said halite pieces and converting said aggregates to crystalline sodium chloride particles, all of which have effective diameters less than 2.5 millimeters and not more than 30 weight percent of which have diameters of less than 0.4 millimeters, thereby exposing a major proportion of said sulfate bodies,   (c) attrition-washing said particles by agitating them with a washing brine which is about 80% or more saturated with sodium chloride and contains less than an amount of sulfate as specified below, said agitation being of such character and intensity that said particles make contact with each other and, as a consequence, a major proportion of said exposed bodies are dislodged from said surfaces,   (d) separating those sodium chloride particles larger than 0.5 millimeter in effective diameter from said brine, and from the dislodged sulfate bodies, the dissolved sulfate content of said washing brine and the amount thereof retained on said larger particles, after said separation, being controlled so that the amount of sulfates added to said particles by the retained washing brine is about 0.05 weight percent or less of their sodium chloride content.   
     
     
       2. The method of claim 1 wherein said larger particles, after being washed and separated from said washing brine, are rinsed with and separated from a rinsing brine, the sulfate content of the latter brine and the amount of it retained on the rinsed particles being controlled so that the amount of sodium sulfate added to said particles by the retained brine is about 0.05 weight percent or less of their sodium chloride content. 
     
     
       3. The method of claim 1 wherein said crushing is carried out in such manner as to maximize the proportion of -8+35 mesh material in the crushed halite and said particles are attrition-washed for a period of from about 2 to about 4 minutes. 
     
     
       4. The method of claim 1 wherein said dislodged sulfate bodies are separated from said sodium chloride particles having effective diameters greater than about 0.5 mm during said attrition-washing operation. 
     
     
       5. The method of claim 1 wherein said washing brine contains 0.2 weight percent or less of dissolved sulfates. 
     
     
       6. The method of claim 2 wherein said rinsing brine contains a total of 0.2 weight percent or less of solutes other than sodium chloride. 
     
     
       7. The method of claim 2 wherein said rinsing brine is saturated in sodium chloride. 
     
     
       8. The method of claim 6 wherein said rinsing brine is saturated in sodium chloride. 
     
     
       9. The method of claim 2 wherein said rinsing brine is more than 85% but less than 100% saturated in sodium chloride. 
     
     
       10. The method of claim 3 wherein said dislodged sulfate bodies are separated from said sodium chloride particles having effective diameters greater than about 0.5 mm during said attrition-washing operation and said larger particles are rinsed with a brine which is about 80% or more saturated in sodium chloride and contains a total of about 0.2 weight percent or less of other solutes.